1. Field of the Invention
The present invention relates in general to a hybrid-vehicle drive system, particularly to a hybrid-vehicle drive system having an engine and an electric motor which are mechanically connected to each other by a planetary gear device, and more particularly to a control when the vehicle drive mode is switched from a motor-drive mode to an engine-drive mode.
2. Discussion of Related Art
There is known a drive system for a hybrid automotive vehicle, which includes (a) an engine operated by combustion of a fuel to produce a drive force, (b) an electric motor, (c) an output member operatively connected to drive wheels, (d) a planetary gear device associated with the engine, electric motor and output member, for synthesizing and distributing a drive force or forces, and (e) a plurality of frictional coupling devices such as clutches and brakes for selectively connecting and disconnecting rotary elements of the planetary gear device to and from each other or a stationary member. An example of such a hybrid vehicle drive system is disclosed in JP-A-9-37411, wherein various vehicle drive modes are selectively established for driving the vehicle, by controlling the operating states of the clutches and brakes. For instance, the drive modes include a motor-drive mode in which the vehicle is driven by the electric motor as a drive power source, and an engine-drive mode in which the vehicle is driven by the engine as the vehicle drive source.
A hybrid-vehicle drive system as described above is not necessarily satisfactory in the manner of control in which the vehicle-drive is switched from the motor-drive mode to the engine-drive mode, when the clutch is engaged while the operating speed of the engine is relatively low. Namely, the known hybrid-vehicle drive system suffers from a reduced vehicle drive force when the clutch is engaged at a relatively low speed of the engine, for switching the vehicle drive mode from the motor-drive mode to the engine-drive mode.